Bad DC offset on my PC-4002m.

[frisouille]

Hello Friends,
I'm actually recapping my pc-4002m. The in circuit board and the two main boards are finished. So when i'd check the bias (set ok) end dc offset, one channel have high offset.; The SM give 0-20mv per channel. Mine have 5.3mv on the left and 245mv on the right channel. What do you advise to check?
Thanks for your help.

 

[Mr. Yamaha]

Isn't there a DC offset pot per channel so that you can adjust it?

 

[avionic]

Isn't there a DC offset pot per channel so that you can adjust it?

I haven't looked at the manual
Most likely not. Probably a DC servo. I'll have to look over the schematic.

 

[frisouille]

No DC servo like many yam pro. IC chips seems doing the job. I think that for this one it's a ti NE5532P.

 

[avionic]

I don't have a scat. But I'd be looking at the input differential pair.

 

[frisouille]

Thanks Avionic. Waiting for your analyze.

 

[avionic]

Waiting for your analyze.

Don't hold your breath.. I don't have a manual or a schematic diagram.

 

[frisouille]

I have one. Lucky i am!!

 

[avionic]

Q201 on the right channel.

 

[avionic]

2SK389BL would be my first suspect.

 

[frisouille]

Thanks Avionic. I'll test him tomorrow. If it's shorted, which equivalent will go for it?

 

[frisouille]

Euh, after looking, i don't know how to test it!!

 

[avionic]

Thanks Avionic. I'll test him tomorrow. If it's shorted, which equivalent will go for it?

2SK389BL -- If it was shorted it would probably trigger the protect circuit.

 

[frisouille]

The protection circuit seems to run fine.

 

[frisouille]

And sometimes the offset change and be ok for a few seconds.

 

[brutal]

Thing about those n-Channel JFET is they will "work" but can be squiffy.

Basic test here:

You can build a test rig and watch this video for further insight.

Around 8:45 he shows the schematic for the test rig and connections.

Disclaimer, I haven't built this myself or put a JFET under test like this so you might want to find a sacrificial N-Channel JFET (doesn't have to be the dual JFET in your amp) to experiment on first.

 

[frisouille]

2 questions. If one 2sk389 is out, should i have to change the two channels, cause i've see that this j-fet are matched by pair?
I have a mk-168 transistors tester, does it can hlp me for testing the j-fet?
Thanks.

 

[avionic]

If one 2sk389 is out, should i have to change the two channels,

Your matched pair is on the single chip (2SK389). So, no you would not need to mess with the other channel.

 

[avionic]

I have a mk-168 transistors tester, does it can hlp me for testing the j-fet?

I don't know. I'd have to read the owners manual.

 

[avionic]

I have a mk-168 transistors tester, does it can hlp me for testing the j-fet?

If its anything like my DCA. About all it will tell you is its a n-channel or p-channel jfet. Or if its shorted or unidentifiable component.